Creating machine direction (MD) elongation in sanitary tissue products such as bath tissue, paper towels, and facial tissue, that comprise fibrous structures that comprise pulp fibers, especially wet-laid fibrous structures, has been a challenge for formulators. Formulators have been limited to creating MD elongation in such sanitary tissue products primarily by one way; namely, foreshortening the fibrous structures during the fibrous structure making process, for example papermaking process. Foreshortening includes both process induced foreshortening and structure induced foreshortening. Both process induced foreshortening and structure induced foreshortening operations build MD elongation into the fibrous structure during the fibrous structure making process.
Process induced foreshortening operations include wet microcontraction operations and/or rush transfer operations, which include fabric creping and/or belt creping operations, creping operations that crepe the fibrous structure off a drying cylinder, for example off a Yankee, and microcreping operations, such as passing the fibrous structure through a microcreper, for example a microcreper commercially available from Micrex Corporation. The effect of process induced foreshortening is the generation of ridges, oftentimes referred to as “crepe ridges” especially those resulting from creping the fibrous structure off a drying cylinder. The limitations and placement of the process induced foreshortening operations results in the ridges being substantially oriented along the cross machine direction (CD) in the resulting fibrous structure. Further, process induced foreshortening negatively impacts the tensile strength, especially the MD tensile strength of the fibrous structure. Due to these negatives associated with process induced foreshortening operations, there is a desire to at a minimum not increase and even reduce the % foreshortening imparted to a fibrous structure by process induced foreshortening operations, but to do so requires foreshortening to be imparted to the fibrous structure by other ways, such as structure induced foreshortening, in order to maintain and/or increase the MD elongation of the fibrous structure.
Formulators have foreshortened fibrous structures during the fibrous structure making process by other process induced foreshortening operations such as wet microcontraction and/or rush transfer, where the fibrous structure is transferred from an upstream operation that is running at a faster speed than a downstream operation, to build MD elongation in the fibrous structures. For example, a rush transfer operation may include a forming wire in a fibrous structure making process running at a faster speed than a transfer fabric and/or through-air-drying fabric, such as is found in an uncreped through-air-dried (UCTAD) process, onto which the fibrous structure is transferred from the forming wire. In another example, a creping roll in a fabric and/or belt crepe operation may be run at a faster speed than the fabric and/or belt that receives the fibrous structure from the creping roll. In still another example, a fibrous structure may be creped (% crepe) from a drying cylinder (i.e., a Yankee) by a doctor blade wherein the drying cylinder is moving at a faster speed than the doctor blade, which is typically stationary in the machine direction. All of these operations are process induced foreshortening operations. For purposes of the present invention, total foreshortening (TFS)=total process induced foreshortening=wet microcontraction+rush transfer+% crepe+microcreping. Process induced foreshortening of a fibrous structure being made generates MD elongation in the fibrous structure. However, the amount of total foreshortening and thus MD elongation resulting from process induced foreshortening that can be imparted to a fibrous structure during the fibrous structure making process has a limit. Therefore, formulators have looked at different ways to build MD elongation into fibrous structures, for example by looking at different through-air-drying fabric and/or patterned belt designs to impart additional MD elongation, with or without process induced foreshortening operations, to the fibrous structures during structure induced foreshortening operations.
Structure induced foreshortening operations include forming and/or drying a fibrous structure on a through-air-drying fabric and/or a patterned through-air-drying belt (a belt that has a three-dimensional material, such as polymer resin in a discrete, semi-continuous, and/or continuous pattern of protuberances or knuckles, which define a discrete, semi-continuous, and/or continuous pattern of deflection conduits or pillows in areas that are void of the protuberances (knuckles). The deflection of the fibrous structure into the deflection conduits (pillows, lower fiber density region) in the patterned belt generates knuckle (non-deflected—higher fiber density region) and pillow pattern to the fibrous structure during the fibrous structure making process which coupled with the pattern results in foreshortening of the fibrous structure. Unlike process induced foreshortening operations, structure induced foreshortening operations do not negatively impact or not as significantly, the tensile strength, especially the MD tensile strength of the fibrous structure made.
To date, formulators have fallen short of making sanitary tissue products comprising a fibrous structure comprising a plurality of pulp fibers that exhibit consumer desired MD elongation properties per total foreshortening (total process induced foreshortening). For all else equal, fibrous structures comprising pulp fibers that exhibit a high MD elongation/TFS will exhibit a higher tensile strength. Formulators have made sanitary tissue products employing fibrous structures that have been made on various through-air-drying fabrics and/or patterned belts without achieving a consumer desired MD elongation to total foreshortening (total process induced foreshortening) ratio. In one example, a prior art sanitary tissue product employing a fibrous structure made using a patterned through-air-drying belt having a surface pattern shown in prior art FIG. 1, where the line elements were substantially oriented in the machine direction, exhibited a MD elongation to total foreshortening (total process induced foreshortening) ratio of 2.22 or less. Other sanitary tissue products employing fibrous structures that have been made on patterned through-air-drying belts having a continuous knuckle pattern and a process induced foreshortening of 0 or greater exhibited MD elongation to total foreshortening (total process induced foreshortening) ratios of 2.43 or less. Further, other sanitary tissue products employing fibrous structures that have been made on various through-air-drying fabrics, which obviously are limited in their design due to the nature of the warp and weft of the fabrics, exhibited MD elongation to total foreshortening (total process induced foreshortening) ratios of less than 2.22. In addition, sanitary tissue products employing fabric creped and belt creped fibrous structures and uncreped through-air-dried fibrous structures all have exhibited MD elongation to total foreshortening (total process induced foreshortening) ratios of less than 2.22. Therefore, the problem addressed by the present invention is how to generate increased MD elongation in a fibrous structure during a fibrous structure making process, for example a papermaking process, using structure induced foreshortening, for example using a new through-air-drying fabric design and/or new patterned through-air-drying belt design such that a sanitary tissue product employing the fibrous structure exhibits a consumer desired MD elongation to total foreshortening (total process induced foreshortening) ratio; namely, a MD elongation to total foreshortening (total process induced foreshortening) that is greater than such ratio in known sanitary tissue products.
Accordingly, there is a need for a sanitary tissue product that exhibits a MD elongation to total foreshortening (total process induced foreshortening) ratio that is greater than such ratios in known sanitary tissue products, and methods for making such sanitary tissue products.